1. Field of the Invention
The present invention relates to a charge/discharge control circuit and a charging type power supply device which are capable of controlling charge/discharge of a battery unit including a plurality of secondary batteries connected in series, and more specifically, to a charging type power supply device with enhanced safety.
2. Description of the Related Art
Enhancing safety is one of the important issues for charging type power supply devices. In particular, the charging type power supply device equipped with a plurality of secondary batteries involves such a problem that charge/discharge control may fail due to disconnection of a connecting section between a battery unit and a charge/discharge control circuit (hereinafter, referred to as “intermediate terminal disconnection”). Taking measures against the intermediate terminal disconnection, a charging type power supply device as illustrated in FIG. 3 is known.
The charging type power supply device of FIG. 3 is configured as follows. Secondary batteries 2 and 3 are inserted in series between a positive power supply terminal 4 and a negative power supply terminal 6 of a charge/discharge control circuit 1. A connection point between the secondary batteries 2 and 3 is connected to an intermediate terminal 5 of the charge/discharge control circuit 1. A voltage of the secondary battery 2 is divided by a voltage dividing circuit 7, and the divided voltage is detected by a voltage detection circuit 9. An output of the voltage detection circuit 9 is supplied to a control circuit 11. When any one of the secondary batteries is over-charged or over-discharged, the control circuit 11 outputs a signal for turning OFF a charge/discharge control switch (not shown) provided between the secondary batteries and an external terminal of the charging type power supply device. Accordingly, the control circuit 11 is formed only of a logic circuit. Also for the secondary battery 3, an overcharge state or an overdischarge state is detected by means of a voltage dividing circuit 8 and a voltage detection circuit 10. A result of the detection is supplied to the control circuit 11 in the form of digital signal similarly. Therefore, when any one of the secondary batteries 2 and 3 is over-charged or over-discharged, the control circuit 11 operates to interrupt the connection between the secondary batteries and the outside so that the progress of overcharge or overdischarge halts. Those two secondary batteries have charging characteristics and discharging characteristics which are not exactly the same, and hence their individual detection controls on overcharge/overdischarge are necessary.
A constant current circuit 14 allows a constant current to flow from the positive power supply terminal 4 to the intermediate terminal 5. The constant current circuit 14 uses a depletion type metal oxide semiconductor (MOS) transistor in which a gate terminal and a source terminal are connected.
The conventional charging type power supply device is configured as described above and operates as follows to detect the intermediate terminal disconnection and stop charging/discharging. When the intermediate terminal 5 and the secondary batteries 2 and 3 are disconnected, the constant current of the constant current circuit 14 flows into the voltage dividing circuit 8, and accordingly a voltage of the intermediate terminal 5 is pulled up to that of the positive power supply terminal 4. Applied with a voltage higher than normal, the voltage dividing circuit 8 detects the overcharge. Therefore, the control circuit 11 outputs the signal for turning OFF the charge/discharge control switch.
In other words, because the constant current circuit 14 is provided between the positive power supply terminal 4 and the intermediate terminal 5, even if the intermediate terminal 5 is disconnected, the secondary batteries are prevented from being over-charged or over-discharged. Therefore, a charging type power supply device with high level of safety may be provided (see, for example, Japanese Patent Application Laid-open No. Hei 08-308115).
However, there is a problem in that it is difficult for the conventional constant current circuit to set a current value necessary and sufficient to detect the intermediate terminal disconnection.
Under the intermediate terminal disconnection situation, the potential of the intermediate terminal is fixed by means of the voltage dividing circuits connected at its upstream and downstream. For that reason, it is necessary that the constant current circuit pull up the intermediate terminal with a current value exceeding a current value of the voltage dividing circuits. Because the voltage dividing circuit is formed of resistive elements, the setting of the constant current value needs to be made taking into account manufacturing fluctuations in resistive elements, with the result that a large redundant constant current is necessary.
Meanwhile, the current of the constant current circuit is an unbalance current for the batteries connected, and hence a too large constant current is responsible for unbalancing the battery voltages.